Motor-control.



E. F. W. ALEXANDERSON.

MOTOR CONTROL.

APPLICATION FILED AUG.10,1907.

957,454, Patented May 10, 1910.

W Ernst fiwAlexandcrson UNITED STATES PATENT OFFICE.

ERNST F. W. ALEXANDERSON, OF SCHENECTADY, NEW YJRK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION 0] NEW YORK.

MOTOR CONTROL.

To all whom it may concern:

Be it known that I, ERNST F. W. ALExaN- DERSON, a citizen of the United States, residing at Schenectady county of Schenectady,

State of New Yorlr, have invented certain new and useful Improvements in Motor Control, of which the following is a specification.

My invention relates to the control of alternating-current series motors, and its object is to improve the ampere speed characteristic of such motors. The torque and current ofan alternating-current motor fall off less rapidly with increase of speed than in the direct-currentmotor, so that an alternating-current motor designed for the same starting torque as a direct-current motor tends to overload itself at high speeds.

My invention consists in a novel connection for such motors adapted to produce automatically a gradually increasingshunt excitation as the motor speeds up. This re sults in strengthening the motor field for high speeds, thereby reducing the speed for a given current and torque, and so preventing the motor from overloading itself at high speeds.

:Oli a car or locomotive, where two or lnOlP motors are employed, a convenient way of btaining an automatic gradual increase of ,he shunt excitation is to produce a phasedisplacement between the voltages at the armature terminals of a pair of motors, and impressing on the motor fields a shunt voltage corresponding in amount to this phase displacement. This phase-displacement increases with increase of speed, so that if a voltage proportional to the phase-displaceimpressed on the motor terminals, and necessarily lags nearly ninety degrees behind the shunt voltage that produces it, as the field winding is highly inductive.

a My invention will best be understood by reference to the accompanying drawings, in which A Figure 1 shows diagrammatically a-pair Specification of Letters Patent.

Application filed August 10, 1907.

with each other, while the field windings Patented May 10, 1910. Serial No. 388,001.

of motors arranged and connected in accordance with my invention; and Figs. 2 and 3' are explanatory diagrams.

In Fig. 1, A A represent the armatures of two series motors, B B their fields, and C C the compensating windings ordinarily used in alternating-current series motors. Since the compensating windings serve simply to improve the efiiciency and power factor of the motors by neutralizing armature reaction and self-induction, and are not essential to my invention, I shall for the sake of convenience, omit further reference to the compensating windings. The two armaturcs are connected directly in series BB are connected in parallel with each other and in series with the armatures. This parallel connection of the fields is made for reasons which will hereinafter appear, but it has an additional and incidental advantage that may be noted in passing, since it makes it possible to obtain in a simple man= nor the stronger fields required. for directcurrent operation, simply by changing the fields from parallel to series in changing from alternating to direct-current operation. D represents the supply transformer. Either the primary or secondary win'ding may be variable as to the number of its turns, as is indicated by the movable contact (Z. One terminal of the armature circuit, that is, the upper terminal of compensating winding 0, is connected to point 1 on the secondary winding of transformer D. The free terminals of field windings B and B are connected to points 2 and 3, respectively, on the transformer winding. The common terminal of the two fields is connected to the armature circuit at. 4. 5 represents the point of connection between the two armatures. E represents a transformer having a 1-to-2 ratio, the primary being connected between point 5, which is the point of connection between the two motor armatures, and point 6 011 the sec ondary winding of the transformer D, while the secondary winding of transformer E is connected between point 4, the common terminal of the field windings, and point 7 on the secondary of transformer D, midway between points 2 and 3, to which the other terminals of windings B and B are connected. The point 6 is so selected that it is' half way between points 1 and 7. The efitects of these connections are shown in Figs. 2- and 3, of which Fig. 2 shows the lowspeed conditions, and Fig. 3 the high-speed conditions. In these figures line l- 2 represents in length and position the amount and phase of the secondary voltage of the supply transformer D. If both fiel d windings B and B were connected to point 7 on the supply transformer, the line -l'[ in Fig. 2 would represent the voltage-dro across the fields, and the line l-d the ve tage-drop across the armatnres in series, and the voltages at the armature terminals of the two motors would then be in phase. the angle 147 is shown as a right-angle, since the armature voltages are in phase with the field-current, which is approximately ninety degrees behind the voltage across the field terminals, as the field winding is highly inductive. Since the field windings B and B are not connected to the same point on the siipply-transforiner. but are connected to the points 2 and 3, displaced from each other, the triangle 1t7 does not represent the actual conditions.

Owing to the displacement between the field connections, the voltages across the terminals of the two fields B and Bare represented by the lines -l2 and elf-3, respee tively, while the voltages across the armature terminals X and A are represented by the lines 15 and 54, respectively; which lines are respectively perpendicular to the lines 41- 2. and t-S. The voltage across the primary terminals of transformer E is consequently represented by the line 56, while the voltage across the secondary terminals is represented by the line 4C7. The transformer E, as has already been said, has a ratio of l to 2, and the line 1-(5 is equal to the line 6-T, so that if both field Windings were connected to the point 7, and there were consequently no phase displacement between the armature voltages, the primary and secondary voltages at the terminals of transformer I) would be balanced, but owing to the phasedis 'ilacement. of the armature voltages, as shown in Fig. 2, the voltage 5 m across the primary terminals of transl'ornmr I) is more than half the voltage iti across its secondary terminals. Consequcntly, there a flow of energy into the primary winding of the transformer and a di-elivery of energy from the secondary of the transformer 19, which secondary is connected in shunt to the field windings B and B; [11 other words, the tra1'1s'l.or1ner E supplies to the licld windings B and B a v hunt excitation which depends in amounton the phasc-displaccnlent between the voltages at the terminals of the two armatures.

lug. 2, as has been said, represents the lowspeed conditions when the voltage-drop across the field windings is proportionately greater than at high speeds. Fig. 3 represents the high-speed conditions, in which the current, and consequently, the voltage-drop across the field windings is reduced, and the voltage across the armature terminals of the meters is increased. Since points 3 and 2 are fixed, a reduction in the length of the lines l-3 and l2 increases the angle 3+2, and since the lines 15 and 5t remain perpendicular, respectively, to the lines 4-2and 4 -3, the angle l5 l becomes smallcr, or, in other words, the phasedisplacement between the voltages at the armature terminals of the two motors increases. This means that the point of intersection 5 between lines l5 and L S moves away from the line lt. The position of this point, and consequently the length of line 5*(5, depends upon the phase relations of the voltages at the armature terminals of the two motors, and therefore upon the angle between the lines and 24c; while the length of the line 17 depends simply upon the drop in the field windings of the motors, and consequently upon the motor current. In Fig. 3 the line 56 is not only greater than one-halt oi the line -l:-7, but is greater than the whole of the line t-7, so that a comparatively great interchange of energy takes place between the windings of transformer lt,-or, in other words, a com paratively strong shunt excitation is impressed on the field windings.

In order to make the diagrams of Figs. 2 and 3 clear, the voltage-drop across the field windings has been considerably exaggerated. in other words. the angle l-12 has been shown greater than it would actually be in p. 'actice. This means that in practice the line 'l7, and consequently the line 5--($, would be more nearly perpendicular to l-2 than appears in these figures. But even in these figures it will be seen that, at high speeds especially, these lines make an angle of nearly ninety degrees with the line 1-2, which represents the phase of the voltage impressed on the motor terminals. words, the shunt voltage impressed on the fields is nearly ninety degrees out of paase with the impressed motor voltage; so that the fieldcurrent due to this voltage, which lags ninety degrees behind it. is approximately in phase with the impressed motor voltage, which is the proper phase for the shunt field current in order that it may be effective in producing a counter-clectromotive force in the armaturcs nearly in opposition to the impressed motor voltage.

In the above description no mention has been made of the drop across the compensating windings. Since the compensating windings are in inductive relation to the armatures, the voltage-drop across them is little greater than that due to the ohmic resistance, and is consequently ne ligible. For that reason the armature and compen- In her sating windings have been treated as a unit in the above discussion, and it will be un derstood that while in any efiicient alternating-current series motor a compensating winding is necessary, the use of such a windin forms no part of my present invention.

at I claim as new, and desire to secure by Letters Patent of the United States, is,-

1. In combination with a plurality of alternating current motors each having its field and armature connected in series, connections for deriving from the motors a voltagedisplaced in phase nearly 90 degrees from the terminal voltage of the motors and increasing gradually and automatically as the motors speed up and connections including means for impressing on the motor fields a shunt voltage corresponding in amount and phase to said derived voltage.

2. In combination with a plurality of alternating current series motors connected in series, connections for producing a phasedisplacement between the vvoltages at the armature terminals of the motors, and connections including means for impressing on the motor fields a shunt voltage corresponding in amount to said phase-displacement.

3. In combination with a plurality of alternating-current' series motors connected in series, connections for producing a phase-displacement between the voltages at the armature terminals of the motors gradually and automatically increasing with increase of speed, and connections including means for impressing on the motor fields a shunt voltage corresponding in amount to said phasedisplacement.

4. In combination with a plurality of alternating-current series motors connected in series, a supply circuit therefor, connections for producing a phase-displacement between the voltages at the armature terminals of the motors, and connections including means for impressing on the motor fields a shunt voltage corresponding in amount to said phase-displacement and displaced in phase approximately ninety degrees from the voltage of the supply circult.

5. In combination with a plurality of alternating-current series motors connected in series, a supply circuit therefor, connections for producing a phase-displacement between the voltages at their armature terminals gradually and automatically increasing with increase of speed, and connections including means for impressing on the motor fields a shunt voltage corresponding in amount to said phase-displacement and displaced in phase approximately ninety degrees from the voltage of the supply circuit.

(5. In combination with a pair of alternating-current motors having their armatures in series and their fields in parallel with each other and in series with the armatures, a winding for supplying current to said motors, the free terminals of the fields being connected to points on said winding displaced from each other, and a'transformer having its primary connected between the point of connection of the two armatures and a point on the supply-winding and its secondary in shunt to the field windings.

7. In combination with a pair of alternating-current motors having their armatures in series and their fields in parallel with each other and in series with the 'armatures, a winding for supplying current to said motors, the free terminals of the fields being connected to points on said winding displaced from each other, and a transformer having its primary connected between the point of connection of the two armatures and a point on the supply winding and its secondary connected between the common terminal of the fields and a point on said supply winding between the points to which the other field terminals are connected.

8. In combination with a pair of alternating-current series motors connected in series, a winding for supplying current thereto, connections for producing a phase displacement between the voltages at the armature terminals of .the two motors, and a transformer having its primary connected between the point of connection of the two motor armatures and a point on the supplywindin and'its secondary in shunt to the motor elds.

In witness whereof, I have hereunto set my hand this 9th day of August, 1907.

ERNST F. W. ALEXANDERSON.

Vitnesses:

BENJAMIN B. HULL, HELEN ORFORD. 

